If used, the test check shall determine the proportion that taxable retail sales bear to all of the vendor's retail sales and the ratio which the tax required to be collected under sections 5739.02, 5739.021, 5739.023, and 5739.026 of the Revised Code bears to the receipts from the vendor's taxable retail sales.
The vendor's liability for remitting the tax shall be based solely upon the proportions and ratios established in the agreement until such time that the vendor or the commissioner believes that the nature of the vendor's business has so changed as to make the agreement no longer representative. The commissioner may give notice to the vendor at any time that the authorization is revoked or the vendor may notify the commissioner that the vendor no longer elects to report under the authorization. Such notice shall be delivered to the other party in the manner provided in section 5703.37 of the Revised Code. The revocation or cancellation is effective the last day of the month in which the vendor or the commissioner receives the notice.
R.C. § 5739.05